Linux Bridge interfaces can be used like VLANs to logically isolate traffic within a Proxmox host. For example, on a LAN, you might create a VLAN for a DMZ and an internal LAN network. In Proxmox, you can create one or more secondary Linux Bridge interface(s) that VMs can be attached to, like a physical interface. Unlike VLANs though, traffic is not tagged. It is logically isolated through a virtual switch. A virtual router is required in this setup.

To add a Linux bridge interface, click on the node (1), then click on Network (2). Click on the Create menu (3). Finally, click on Linux Bridge (4).

The new Linux Bridge window opens. At this point, you can adjust the name of the Linux Bridge interface, if you want. By default, Proxmox will choose the next bridge number. No adjustments need to be made on this window other than a comment if you wish. Click on Create.

Next, you will need to add this Linux Bridge interface to your virtual router. To do this, select your router (1) in the list of VMs. Click on the Hardware (2) option. Click on the Add (3) button. Click on Network Device (4).

Next, the Add Network Device window will open. Select the bridge you want to add (1) from the list (2). Click on Add (3).

You will now see the Linux Bridge interface added to your virtual router.

At this point, you can configure your router or firewall to use this interface as well as any firewall rules. Consult your router or firewall vendor's documentation on how to do this.

If you want a VM to be on a specific internal network, you will need to adjust that VM's network interface to use the respective bridge interface.